1. Field of the Invention
The present invention relates to a digital-to-analog converter and source driving circuit, and more particularly to a digital-to-analog converter and source driving circuit requiring fewer switches by reordering reference voltages.
2. Description of the Prior Art
A liquid crystal display (LCD) monitor has characteristics of light weight, low power consumption, zero radiation, etc. and is widely used in many information technology (IT) products, such as televisions, mobile phones, and laptop computers. The operating principle of the LCD monitor is based on the fact that different twist states of liquid crystals result in different polarization and refraction effects on light passing through the liquid crystals. Thus, the liquid crystals can be used to control amount of light emitted from the LCD monitor by arranging the liquid crystals in different twist states, so as to produce light outputs at various brightnesses.
Please refer to FIG. 1, which is a schematic diagram of a thin film transistor (TFT) LCD monitor 10 of the prior art. The LCD monitor 10 includes an LCD panel 100, a source driver 102, a gate driver 104 and a voltage generator 106. The LCD panel 100 is composed of two substrates, and space between the substrates is filled with liquid crystal materials. One of the substrates is installed with a plurality of data lines 108, a plurality of scan lines (or gate lines) 110 and a plurality of TFTs 112, and another substrate is installed with a common electrode for providing a common signal Vcom outputted by the voltage generator 106. The TFTs 112 are arranged as a matrix on the LCD panel 100. Accordingly, each data line 108 corresponds to a column of the LCD panel 100, each scan line 110 corresponds to a row of the LCD panel 100, and each TFT 112 corresponds to a pixel. Note that the LCD panel 100 composed of the two substrates can be regarded as an equivalent capacitor 114.
The source driver 102 and the gate driver 104 input signals to the corresponding data lines 108 and scan lines 110 based upon a desired image data, to control whether or not to enable the TFT 112 and a voltage difference between two ends of the equivalent capacitor 114, so as to change alignment of the liquid crystals as well as the penetration amount of light. As a result, the desired image data can be correctly displayed on the LCD panel 100. In order to display various gray levels, the source driver 102 has to provide hundreds of voltage levels to the LCD panel 100. For example, to display 256 gray levels, the source driver 102 has to select one reference voltage from 256 reference voltages according to an 8-bit digital signal to be a source driving signal. However, implementation for the 256-level selection requires a large number of transistor switches, which occupy a large circuit layout area and cause parasitic resistors, which is a heavy burden on display driving efficiency.